1. Field of the Invention
The present invention relates to a magnetic recording medium with a high recording density and excellent recording characteristics in a wide wavelength range from a short wavelength to a long wavelength.
2. Description of the Related Art
Conventionally, a coated type magnetic recording medium is produced by coating a magnetic powder such as a ferrite or iron powder on a base material such as a polyester film along with a resin binder. This resin binder is added so as to improve the dispersion property of the magnetic powder and the running durability of the magnetic recording medium. It is known that the amount of resin binder to be added to a magnetic powder of 100 parts by weight is at least 10 parts by weight.
Recently, needs of magnetic recording media with high recording density have been increasing. As one of the methods for satisfying such needs, the particle diameter of the magnetic powder is decreased and the coercive force (Hc) thereof is strengthened. As fine-particle magnetic powders employable for high density recording, a metal powder whose particle diameter is 0.3 .mu.m or less, an ultra-fine hexagonal powder such as barium ferrite and others are suitable. This is because the above mentioned fine-particles of magnetic powders are so arranged in the magnetic layer, which is made by smoothly coating the particles on the base material with a high packing ratio, that magnetized direction of them are perpendicular to the medium surface and not magnetically repulsive one another. Thus, this type of magnetic recording media is suitable for high density recording.
However, although a magnetic layer containing a fine magnetic powder with a high coercive force has high reproduction output in a short wavelength range, the reproduction output in a long wavelength range is inferior to that of the conventional medium containing needle shaped .gamma.-ferrite. To solve such a problem, a two-layer coated magnetic recording medium has been proposed. The two-layer type coating magnetic recording medium comprises a non-magnetic base material, a lower magnetic layer containing a magnetic powder with a relatively low coercive force suitable for recording in a long wavelength range, and an upper magnetic layer containing a magnetic powder with a small particle diameter and a high coercive force suitable for recording in a short wavelength range. The lower magnetic layer is disposed on the non-magnetic base material, the upper magnetic layer being disposed on the lower magnetic layer. Thus, this two-layer coated magnetic recording medium can deal with a wider wavelength range.
To satisfactorily obtain the characteristics of this two-layer coated medium, it is preferable to set the thickness of the upper magnetic layer to an order of submicrons. To do this, it is necessary to decrease as small the surface roughness of the upper magnetic layer as possible, thereby improving the smoothness of the surface. Thus, it is also necessary to decrease as low the surface roughness of the lower magnetic layer as possible, thereby preventing the surface property of the lower magnetic layer from negatively affecting that of the upper magnetic layer.
In addition, to improve the reproduction output of the magnetic recording medium, themagnetic particles of the magnetic coating materials composing the upper magnetic layer should be dispersed into the resin binder in the state that each magnetic particle is separated almost individually, i.e., in the state of almost primary particles. Moreover, the packing ratio of the magnetic particles to the resin binder should be as high as possible.
However, in the conventional two-layer coated medium, it has been difficult to set the coating thickness of the upper magnetic layer to an order of submicrons. This is because the surface property of the lower magnetic layer negatively affects that of the upper magnetic layer. Moreover, disturbance which takes place at the interface between the lower magnetic layer and the upper magnetic layer has resulted in increase of noise in recording and reproducing. These problems have been especially remarkable when the lower magnetic layer and the upper magnetic layer are coated on the base material at the same time. For example, when the upper magnetic layer having a coating thickness of 0.5 .mu.m or less has been formed, part of the lower magnetic layer has been occasionally extruded to the upper magnetic layer.
The "substantially at the same time" means that the upper magnetic layer is coated before the organic solvent of the coating material for the lower magnetic layer is not evaporated and dried. Thus, even if these layers are not coated at the same time, but with a slight delay, it is construed that these layers should be coated at the same time.
The surface property of the upper magnetic layer and that of the interface between the upper magnetic layer and the lower magnetic layer is remarkably affected by the type and amount of various organic or inorganic additives such as carbon black and an abrasive which are used along with the resin binder.
For example, the abrasive has important functions for improving the running durability of a medium and for preventing a head from clogging. The abrasive is conventionally made of inorganic particles with high hardness and average particle diameter of 0.5 .mu.m to 1.0 .mu.m or more (defined with a median diameter on a weight cumulative particle distribution curve). However, when a coating material containing an abrasive with a particle diameter of 0.5 .mu.m or more for use with the upper magnetic layer has been coated along with a coating material for use with the lower magnetic layer, it has been difficult to decrease the thickness of the upper magnetic layer to 0.5 .mu.m or less. This is because the surface of the lower magnetic layer which has been coated is remarkably disturbed by the particles of the abrasive contained in the coating material for use with the upper magnetic layer.
To prevent such a problem, the coating material for use with the lower magnetic layer has been coated in advance and then dried. Thereafter, the coating material for use with the upper magnetic layer was coated on the dried film. However, in this case, another problem has taken place. In other words, the particles of the abrasive has extruded to the surface of the upper magnetic layer, thereby damaging the recording and reproduction head while the medium running.
To prevent the extrusion of the particles of the abrasive, it has been possible to use an abrasive composed of small particles. However, to achieve the same effect by means of the abrasive with a small particle diameter, the amount of abrasive to be added to the medium had had to be increased. When the amount of abrasive is increased, the packing ratio of the magnetic powder is decreased. Thus, the reproduction output of the medium were remarkably deteriorated.
Furthermore, as the size of magnetic powder particles for use with the upper magnetic layer is so small, it has been becoming difficult to mix these particles with resin binder so thoroughly that each particle are separated individually. Likewise, it has been becoming further difficult to decrease the amount of resin binder, so as to increase the packing ratio of the magnetic powder, and it has also becoming more difficult to decrease the surface roughness. As the size of the magnetic powder particles is smaller, it is technically more difficult to improve the reproduction output of the magnetic recording medium.
In other words, according to the needs of high recording density of the magnetic recording media, the dispersion techniques of ultra-fine magnetic powders for use with the upper magnetic layer have not been satisfactorily established. In addition, the smoothness of the coated surface and the packing density of the magnetic powder of the upper magnetic layer have not been adequately improved. As a result, the reproduction output level in a short wavelength range has not achieved the level as is expected from the particle diameter of the magnetic powder for use with the upper magnetic layer. Moreover, when the magnetic powder for use with the upper magnetic layer has been dispersed in sufficiently, noise in recording and reproducing has been increased. These problems resulted in difficulties when recording in a short wavelength range.